Don't underline all changes in plaintext files

Fixes #371

sample_files/compare.expected

@@ -8,7 +8,7 @@ sample_files/change_outer_before.el sample_files/change_outer_after.el
 1857b63ba1bfa0ccc0a4243db6b1c5c2  -
 
 sample_files/chinese_before.po sample_files/chinese_after.po
-d648d62cf864d452a4e862e7eb848be0  -
+56f0af341fd86727dbac522293e8e013  -
 
 sample_files/clojure_before.clj sample_files/clojure_after.clj
 b916e224f289888252cd7597bab339e6  -
@@ -158,7 +158,7 @@ sample_files/simple_before.js sample_files/simple_after.js
 b1fe2c184a9a358e314e21aabb0f3cb7  -
 
 sample_files/simple_before.txt sample_files/simple_after.txt
-c2cea3b652daa2bded20c785b9e4803f  -
+4b653ebe89321835c35722dd065cf6a2  -
 
 sample_files/slider_at_end_before.json sample_files/slider_at_end_after.json
 1d6162aab8e59c4422e9c14f09ceac3e  -
@@ -182,7 +182,7 @@ sample_files/tab_before.c sample_files/tab_after.c
 36ba3231eeba6f0b67a6be9db454de19  -
 
 sample_files/text_before.txt sample_files/text_after.txt
-7abd607b5782061dcd882a3be2dfe48e  -
+dfc3495b8d5931029b479f0c878a3219  -
 
 sample_files/todomvc_before.gleam sample_files/todomvc_after.gleam
 45baae0b84cfc5c1dc91d59be315762b  -

src/display/style.rs

@@ -270,7 +270,7 @@ pub fn color_positions(
     is_lhs: bool,
     background: BackgroundColor,
     syntax_highlight: bool,
-    _language: Option<Language>,
+    language: Option<Language>,
     positions: &[MatchedPos],
 ) -> Vec<(SingleLineSpan, Style)> {
     let mut styles = vec![];
@@ -321,7 +321,14 @@ pub fn color_positions(
                 }
             }
             MatchKind::NovelWord { highlight } => {
-                style = novel_style(style, is_lhs, background).bold().underline();
+                style = novel_style(style, is_lhs, background).bold();
+
+                // Underline novel words inside comments in code, but
+                // don't apply it to every single line in plaintext.
+                if language.is_some() {
+                    style = style.underline();
+                }
+                
                 if syntax_highlight && matches!(highlight, TokenKind::Atom(AtomKind::Comment)) {
                     style = style.italic();
                 }